The reduction of emissions from vehicle exhaust systems is a well known problem. As the number of vehicles having an internal combustion engine continues to increase, the problem is becoming more severe and despite the introduction of catalytic converter exhaust systems, the exhaust emissions from vehicles fitted with such systems are still relatively high.
In particular exhaust emissions are relatively high during the initial warm-up phase, also referred to as the “cold start”, of an internal combustion engine after starting, especially with regard to the emissions of carbon monoxide, oxides of nitrogen and hydrocarbons. Cold start conditions refer to when the catalytic converter is not operating. For example, this could be on a cold winter day when the temperature is −10° C. or on a summer day when the ambient temperature is 30° C. The catalytic converter must be heated to approximately 250° C. before it becomes operable to convert the combustion by-products of the internal combustion engine. Normal operating temperature is in the 400° C. to 800° C. range. For the internal combustion engine to meet the Federal Test Procedure for the new stringent exhaust requirements, the catalyst must come up to temperature as quickly as possible.
The particularly high exhaust emissions are largely due to the fact that the catalytic converter has not reached its so-called “light-off” temperature, at which the catalyst causes the required catalytic reactions to take place. The light-off temperature can be defined as the temperature at which the catalytic converter reaches 50% conversion. Modern catalyst systems start operating at temperatures of around 200° C. to 300° C.
In order to reduce the quantity of harmful emissions during the initial warm-up phase, a plurality of different solutions has been proposed, many of these solutions being based on shortening the time taken to reach the light-off temperature by raising the temperature in the catalyst as fast as possible. During a cold start, this can be achieved by generating increased heat energy into the exhaust system, which subsequently causes the catalyst to be rapidly heated.
A previously known arrangement for obtaining this reduction in time for the light-off temperature to be reached is one comprising an electrically heated catalyst, which is arranged upstream from the main catalyst. However, this arrangement implies certain drawbacks. Firstly, the cost for a heatable catalyst substrate is considerable. Furthermore, the consumption of electrical energy is relatively high. An additional power supply such as an extra battery may be required in the vehicle. Also, the durability of the electrically heatable catalyst substrate may constitute a problem.
Consequently, there exists a need for a catalytic converter that can accelerate catalyst light-off time without increasing the emissions from the vehicle's exhaust system.